This invention relates to an ion current detecting device for an internal combustion engine which detects an ion current which is produced in the gap between the electrodes of a spark plug when the engine is in the process of combustion.
One such ion current detecting device which has previously been disclosed by the present applicant will be described in accordance with FIG. 5.
In FIG. 5, an ignition coil 1 is provided with a primary winding la and a secondary winding 1b; a power transistor 2 is connected between the primary winding la and ground, to control the flow of primary current; and a spark plug 3 is connected to one of two terminals of the secondary winding 1b where an ignition high voltage is provided. The spark plug burns the gas mixture in the internal combustion engine when the ignition high voltage is applied to it. Further in FIG. 5, a capacitor 4 is connected to the other terminal of the secondary winding 1b which is on the side of positive polarity; a resistor 5 is connected between the capacitor 4 and ground, to convert an ion current into a voltage; a diode 6 is connected in parallel to the resistor 5; a Zener diode 7 is connected between the other terminal of the secondary winding 1b and a coil power source 10; an AC coupler 8 obtains only an AC component from a voltage developed across the resistor 5; a comparator 9 compares a voltage value provided by the AC coupler 8 with a predetermined comparison level; and the output terminal 11a of the comparator 9 supplies a pulse signal when an ion current is detected.
The ion current detecting device thus organized operates as follows:
When, at the time of ignition of the internal combustion engine, the power transistor 2 is rendered nonconductive (off) to cut off the primary current of the primary winding 1a, an ignition high voltage of negative polarity is induced in the secondary winding 1b, so that spark discharges occur between the electrodes of the spark plug 3 to burn the gas mixture in the internal combustion engine. In this operation, as the gas mixture burns, ionization takes place, thus forming ions. In this case, the electrodes of the spark plug 3 act as electrodes for detection of ion current after the occurrence of spark discharges. The bias of positive polarity of the capacitor 4 moves electrons, so that an ion current flows. When the ion current flows in this manner, a voltage is developed across the resistor 5. The AC component of the voltage thus developed is detected by the comparator 9, where it is compared with the predetermined comparison level, as a result of which a pulse signal is provided at the output terminal 11a. The pulse signal is detected to confirm the combustion of the mixture gas.
The conventional ion current detecting device, being designed as described above, is disadvantageous in the following points:
When the ignition coil 1 is excited (on) and unexcited (off), noise pulses are produced in the ion current detecting path by the electro-magnetic induction of the ignition coil 1. That is, with the conventional ion current detecting device, pulses are detected not only when the gas mixture is burnt but also when it is not (misfire). In order to eliminate this difficulty, it is necessary to additionally provide means for determining the ion current from the width of the pulse detected indicating whether or not the gas mixture has been burnt. That is, in the conventional ion current detecting device, the method of determining whether or not the gas mixture has been burnt is rather intricate, and low in reliability.